While many individuals can achieve short term weight loss, long term weight loss maintenance is a challenge for most people. High levels of physical activity are consistently reported by those who succeed in long-term weight loss maintenance. The goal of this proposal is to understand how and why high levels of physical activity are critical for long term maintenance of weight loss. This project takes advantage of the National Weight Control Registry (NWCR), which follows over 6000 individuals who have maintained a weight loss of >30 pounds for >one year. We hypothesize that high levels of physical activity compensate for the reduction in total daily energy expenditure (TDEE) that is the expected result of weight loss, and allow NWCR subjects to achieve energy balance at levels of TDEE similar to pre-weight loss levels. We base this hypothesis on the observations that 1) caloric restriction is effective for weight loss, but usually ineffective as a sole strategy for long term weight loss maintenance;2) high levels of physical activity are consistently associated with success in weight loss maintenance;and 3) high levels of physical activity may improve regulation of food intake. We also hypothesize that physical activity functions to correct an increase in metabolic efficiency (a lower than predicted RMR) in the reduced obese state. We base this hypothesis on observations that 1) RMR is greater in adults who perform regular exercise than their sedentary peers;2) studies in rodents have consistently shown evidence of an increase in metabolic efficiency in the reduced obese state;and 3) studies in humans have shown mixed results, but few studies have controlled for habitual level of physical activity. The specific aims are: 1) To compare TDEE and its components in NWCR subjects successful at long term weight loss maintenance to non-reduced weight matched and obese controls. We hypothesize that TDEE in NWCR subjects is higher than weight matched controls and similar to obese controls. This would suggest reduced obese subjects are using physical activity to achieve energy balance at or near the same level after weight loss as before. We will use doubly labeled water to measure TDEE and total daily activity thermogenesis (TDAT), indirect calorimetry to measure RMR and thermic effect of food, and state of the art physical activity monitoring systems to assess exercise activity thermogenesis (EAT) and non- exercise activity thermogenesis (NEAT). 2) To compare RMR under free living conditions and during an experimentally imposed period of reduced energy flux (restricted physical activity matched by an equivalent reduction in energy intake) in NWCR subjects and controls. We hypothesize that RMR in NWCR subjects will be similar to controls under free living conditions, but will decrease more than controls when activity is controlled to similar low levels. We will achieve a reduction in flux by admitting subjects to the GCRC for 72 hours of bedrest, and providing a diet designed to achieve energy balance in this restricted activity state.